1. Field of the Invention
The invention is directed to a magnetic resonance apparatus.
2. Description of the Prior Art
Magnetic resonance technology is a known technique for acquiring images of the inside of the body of an examination subject. To that end, rapidly switched gradient fields that are generated by a gradient coil system are superimposed on a static basic magnetic field generated by a basic field magnet in a magnetic resonance apparatus. Further, the magnetic resonance apparatus has a radio-frequency system that emits radio-frequency signals into the examination subject for triggering magnetic resonance signals, and that picks up the generated magnetic resonance signals, from which magnetic resonance images are generated.
A superconducting basic field magnet has, for example, an essentially hollow-cylindrical helium container in which superconducting coils are arranged, these being cooled by the liquid helium that surrounds them. The helium container is surrounded by a hollow-cylindrical, inner cryoshield that is in turn surrounded by a hollow-cylindrical, outer cryoshield. The cryoshields allow as little thermal radiation as possible to penetrate to the helium container. To that end, the cryoshields are fashioned of a highly thermally conductive metal, for example aluminum. The cryoshields and/or the helium container are thereby held to prescribed temperatures by a cryo-cooler, cold gas or liquid nitrogen. The outer cryoshield is surrounded by an essentially hollow-cylindrical vacuum container. The containers are usually fashioned of nonmagnetic stainless steel. The helium container is connected to the inner cryoshield, the two cryoshields are connected to one another and the outer cryoshield is connected to the vacuum container with poor thermal conductivity up to a mutual spacing of a few millimeters.
The hollow-cylindrical gradient coil system is secured in the cylindrical cavity of the vacuum container, for example by being force-fit in the cavity. For generating gradient fields, suitable currents are set in the gradient coils of the gradient coil system. The amplitudes of the required currents can be up to several 100 A. The current rise and decay rates can be up to several 100 kA/s. Given an existing basic magnetic field on the order of magnitude of 1 T, Lorentz forces that lead to vibrations of the gradient coil system act on these time-variable currents in the gradient coils.
As discussed in German OS 195 31 216, for example, these vibrations have a number of negative properties, such as acoustic noises that emanate from the gradient coil system and structural noises that emanate from the gradient coil system and are transmitted onto the rest of the magnetic resonance apparatus via the fastenings, as well as image quality disturbances that can be caused by excessive movement of the gradient coil system. German OS 195 31 216 therefore proposes that the gradient coil system be secured in the region of a vibratory node that is to be expected during operation. Disadvantageous influences of vibrations that emanate from the gradient coil system on the rest of the magnetic resonance apparatus are prevented as a result.
Since the gradient coil system is surrounded by conductive structures of the basic field magnet, for example by the steel vacuum container and the outer cryoshield of aluminum, the gradient fields that are switched induce eddy currents in the conductive structures. The fields that accompany the eddy currents are unwanted because they attenuate the gradient fields if counter-measures are not taken, and distort the gradient fields in terms of their time curve, which leads to degradation of the quality of the magnetic resonance images. Further, the eddy currents induced in the conductive structures of the basic field magnet cause an inherently unwanted heating of the basic field magnet. These disadvantageous influences are reduced by utilizing an actively shielded gradient coil system.
An object of the present invention is to provide an improved resonance apparatus wherein, among other things, unwanted eddy current effects are governed better.
This object is achieved in a magnetic resonance apparatus according to the invention having a basic field magnet with an inner unit, an outer unit and at least one middle unit that are essentially hollow-cylindrical and are electrically conductive and that are arranged inside one another such that the inner unit is enveloped by the middle unit and the middle unit is enveloped by the outer unit, and a gradient coil system which causes eddy currents to be induced at least in regions of the outer unit, and wherein at least the inner cylindrical jackets of the units are matched to one another in terms of mechanical properties so that the middle unit effectively damps oscillatory transmission from the outer unit to the inner unit that would otherwise arise due to magnetic coupling.
In the initially described basic field magnet of the prior art, the gradient fields are in fact shielded well from the helium container, particularly by the vacuum container, but the eddy currents produced in the vacuum container due to the gradient fields have accompanying fields that in turn produce eddy currents in the outer cryoshield. Due to the strong basic magnetic field, this leads to a vibratory movement of the outer cryoshield, causing further eddy currents to be generated due to the movement. The fields of the eddy currents occurring in the outer cryoshield in turn induce eddy currents in the inner cryoshield, etc., and the above-described, magnetic coupling propagates to the helium container.
The invention is based on the perception that the above-described oscillatory transmission due to magnetic coupling from the vacuum container in the direction to the helium container is especially pronounced when a mode of characteristic (natural) oscillation that is the same for the inner cylinder jackets of the helium container, of the cryoshields and of the vacuum container has characteristic frequencies (eigenfrequencies) for the individual cylindrical jackets that are approximately the same. With respect to the same mode of natural oscillation, the inner cylindrical jackets behave in a manner comparable to a series circuit of filters with nearly identical passbands with respect to oscillatory transmission due to the magnetic coupling. This is the case given the initially described basic field magnet of the prior art with containers and shields of steel and/or aluminum.
According to the invention, in contrast, at least one inner cylindrical jacket of one of the containers and shields is fashioned such that it exhibits a detuned characteristic frequency compared to the cylindrical jackets of the other containers and shields for an identical mode of characteristic oscillation. The cylindrical jackets thus behave in a manner comparable to a series circuit of filters having different passbands, so that the correspondingly fashioned cylindrical jacket acts as a magneto-mechanical blocking filter that effectively damps the forwarding of oscillations and losses. As a result, the eddy currents are minimized in the helium container, resulting in a lower evaporation rate of the liquid helium due to the slight heating of the helium container induced by eddy currents. Accordingly, time intervals for replenishing the liquid helium are long in an economically advantageous way.